The invention relates to a metal halide lamp having a nominal power of more than 100 W, comprising a discharge vessel with a ceramic wall enclosing a discharge space containing an ionizable filling which, in addition to Hg, comprises a quantity of iodide of Na, Tl, Ho and Ca, and in which two electrodes are arranged, each with their electrode tip located at a mutual distance EA, said discharge vessel comprising a cylindrical part having an internal diameter ID and extending at least through the distance EA.
A lamp of the type described in the opening paragraph is known from WO 98/45872 (N16313). The known lamp has a high specific luminous flux and, in operation, emits light at a high color temperature Tc and a value of at least 90 for the general color rendering index Ra.
In this lamp, use is made of the recognition that a satisfactory color rendition is possible when Na halide is used as a filling constituent of a lamp and when, during operation, there is a strong widening and reversal of the Na emission in the Na-D lines. This requires a high temperature of the coldest spot Tkp in the discharge vessel of, for example, 1170 K (900xc2x0 C.). When reversing and widening the Na-D lines, these assume the shape of an emission band in the spectrum, with two maxima at a mutual distance xcex94xcex.
The requirement for a high value of Tkp excludes the use of quartz or quartz glass for the wall of the discharge vessel and necessitates the use of ceramic material for this wall.
In this description and the claims, a ceramic wall is understood to mean both a wall of metal oxide such as, for example, sapphire or densely sintered polycrystalline Al2O3, and metal nitride, for example, AlN.
Although the known lamp has good color properties and a relatively high luminous flux, the lamp has relatively large dimensions. Concentration of the light to a beam emitted by the lamp, for example, by means of a lens or a diaphragm for projection purposes thus results in a relatively large loss of light. This is a drawback.
It is an object of the invention to provide a lamp of the type described in the opening paragraph, in which the drawback is obviated to a considerable extent.
According to the invention, this object is achieved in that the lamp of the type described in the opening paragraph is characterized in that the relation EA/ID less than 1 is satisfied An advantage of the lamp according to the invention is that the light emitted by the lamp can be better concentrated to a beam, while, surprisingly, it is also possible that the luminous flux and the color temperature Tc decrease in value only to a small extent while maintaining the value for the general color rendering index Ra. The lamp according to the invention is suitable, inter alia, as a light source for coupling light into a light-conducting fiber. A decrease of the ratio EA/ID below 0.65 generally leads, in the case of a constant ID, to an unacceptably low specific luminous flux of the lamp. An improvement of the lamp according to the invention is possible when the lamp has a wall load of at least 110 W/cm2. Wall load is herein understood to mean the quotient of the lamp power and the inner surface of that part of the discharge vessel which extends through the distance EA. It is thereby achieved that, in the case of coupling light into an optical fiber, a better coupling efficiency can be realized, which leads to a higher system efficiency.
In a further variant, the ionizable filling also comprises iodide of Dy and Tm. A lamp with a color temperature Tcxe2x89xa74,000 K is then possible.
The lamp is preferably provided with an outer envelope enclosing the discharge vessel with a space. The space is preferably filled with an inert gas, for example N2. The gas in the space has a cooling effect on the wall of the discharge vessel. In the operating state of the lamp, the pressure of the inert gas is at least 100 Mbar and preferably not more than 1 bar in order that risk of explosion is excluded There is preferably a minimum distance of 3 mm between the outer envelope and the wall of the discharge vessel. At distances of less than 3 mm between the outer envelope and the wall of the discharge vessel, it appears that the wall of the discharge vessel is cooled less effectively. A reduction of the distance below the minimum distance is a drawback in practice because of the positioning of a current conductor to one of the electrodes of the lamp. A furter increase of the distance only results in a small increase of the cooling effect.
A wall thickness of between 0.6 mm and 1.4 mm appears to be advantageous for realizing an optimum temperature distribution across the wall of the discharge vessel. A wall thickness of less than 0.6 mn has the drawback that the temperature of the wall of the discharge vessel will become unacceptably high so that the lifetime of the lamp is influenced detrimentally. Generally, a strong temperature gradient which is unwanted for a desired lifetime of the lamp will also occur across the wall of the discharge vessel. An increase of the wall thickness above 1.4 mm leads to a strong decease of the specific luminous flux.
A suitable temperature gradient across the wall of the discharge vessel is achieved in a preferred embodiment of the lamp according to the invention when the cylindrical part with an internal diameter ID extends between end faces at a mutual distance of at least 2*EA. In a further embodiment, the discharge space enclosed by the discharge vessel is sealed at the area of the end faces.
Preferably, such a value of the coldest spot temperature Tkp is realized that the value for xcex94xcex. ranges between 10 nm and 30 nm.
These and other aspects of the invention are apparent from and will be elucidated with reference to the embodiments described hereinafter.